What are face normals?
Normals are vectors that are Orthogonal to the Tangent plane of a curved surface; Or simply orthogonal to a face.

The Normals are usually used to determine a surface's orientation toward a light source for shading. Most common examples:


Flat, Goraud, Phong shading


Fresnel, Refraction, Reflection effects
Bump, Normal, Displacement mapping


Normals are generally normalized to length 1 (in arbitrary units), otherwise calculation errors, and thus artifacts, may arise. Since they only point one way they are also used for back visibility culling (when double-sided is off).

That's why it is important for the normals to be consistent and coherent (i.e. point in the correct way - outwards of a model).